Electronic display devices, such as monitors used with computers and screens built in to telephones and portable information devices, are usually designed to have a viewing angle as wide as possible, so that they can be read from any viewing position. However, there are some situations where a display that is visible from only a narrow range of angles is useful. For example, one might wish to read a private document using a portable computer while on a crowded train.
Several methods exist for adding a light controlling apparatus to a naturally wide-viewing range display.
One such structure for controlling the direction of light is a ‘louvred’ film. The film consists of alternating transparent and opaque layers in an arrangement similar to a Venetian blind. Like a Venetian blind, it allows light to pass through it when the light is travelling in a direction parallel or nearly parallel to the layers, but absorbs light travelling at large angles to the plane of the layers. These layers may be perpendicular to the surface of the film or at some other angle. Methods for the production of such films are described in a USRE27617 (F. O. Olsen; 3M 1973), U.S. Pat. No. 4,766,023 (S.-L. Lu, 3M 1988), and U.S. Pat. No. 4,764,410 (R. F. Grzywinski; 3M 1988).
Other methods exist for making films with similar properties to the louvred film. These are described, for example, in U.S. Pat. No. 5,147,716 (P. A. Bellus; 3M 1992), and U.S. Pat. No. 5,528,319 (R. R. Austin; Photran Corp. 1996)
Louvred films may be placed either in front of a display panel or between a transmissive display and its backlight to restrict the range of angles from which the display can be viewed. In other words, they make a display “private”.
The principal limitation of such films is that they require mechanical manipulation, i.e. removal of the film, to change the display between the public and private viewing modes.
US 2002/0158967 (J. M. Janick; IBM, published 2002) shows how a light control film can be mounted on a display so that the light control film can be moved over the front of the display to give a private mode, or mechanically retracted into a holder behind or beside the display to give a public mode. This method has the disadvantages that it contains moving parts which may fail or be damaged and that it adds bulk to the display.
A method for switching from public to private mode with no moving parts is to mount a light control film behind the display panel and to place a diffuser which can be electronically switched on and off between the light control film and the panel. When the diffuser is inactive, the light control film restricts the range of viewing angles and the display is in private mode. When the diffuser is switched on, it causes light travelling at a wide range of angles to pass through the panel and the display is in public mode. It is also possible to mount the light control film in front of the panel and place the switchable diffuser in front of the light control film to achieve the same effect.
Switchable privacy devices of these types are described in U.S. Pat. No. 5,831,698 (S. W. Depp; IBM 1998), U.S. Pat. No. 6,211,930 (W. Sautter; NCR Corp. 2001) and U.S. Pat. No. 5,877,829 (M. Okamoto; Sharp K. K. 2001). They share the disadvantage that the light control film always absorbs a significant fraction of the light incident upon it, whether the display is in public or private mode. The display is therefore inefficient in its use of light. Since the diffuser spreads light through a wide range of angles in the public mode, these displays are also dimmer in public mode than in private mode, unless the backlight is made brighter to compensate.
Another disadvantage relates to the power consumption of these devices. In the public mode of operation, the diffuser is switched on so as to be optically diffusing. This often means that voltage is applied to a switchable polymer-dispersed liquid crystal diffuser. More power is therefore consumed in the public mode than in the private mode. This is a disadvantage for displays which are used for most of the time in the public mode.
Another known method for making a switchable public/private display is given in U.S. Pat. No. 5,825,436 (K. R. Knight; NCR Corp. 1998). The light control device is similar in structure to the louvred film described earlier. However, each opaque element in the louvred film is replaced by a liquid crystal cell which can be electronically switched from an opaque state to a transparent state. The light control device is placed in front of or behind a display panel. When the cells are opaque, the display is in its private mode; when the cells are transparent, the display is in its public mode.
The first disadvantage of this method is in the difficulty and expense of manufacturing liquid crystal cells with an appropriate shape. A second disadvantage is that, in the private mode, a ray of light may enter at an angle such that it passes first through the transparent material and then through part of a liquid crystal cell. Such a ray will not be completely absorbed by the liquid crystal cell and this may reduce the privacy of the device.
The concept of using a hologram to provide a privacy function is disclosed in GB2404991 but such a display suffers from two disadvantages. First, due to unwanted diffraction of light from the display by the hologram, the colour of the image seen by viewers is incorrect. Second, for applications using a touch screen mounted on the front of the display, the user's hand can block the illumination of the hologram and so reduce the effectiveness of the privacy. GB 2428128 discloses solutions to these problems.
In GB2410116, a display is switched from public to private mode by using two different backlights which generate light with different angular ranges.
GB2405544 (Sharp) describes switchable privacy devices based on louvres, which operate only for one polarisation of light. The louvres are switched on and off either by rotating dyed liquid crystal molecules in the louvre itself or by rotating the plane of polarisation of the incident light using a separate element.
In GB2421346 (Sharp), a polarisation modifying layer (PML) is placed behind the exit polariser of a liquid crystal display panel. Some parts of the PML are simply transparent. Other parts change the polarisation of light passing through them so that pixels viewed through these parts are inverted in colour (bright pixels becoming dark and dark pixels becoming bright). Data sent to pixels directly behind these parts is inverted so that, when the display is viewed from a central position, the image appears normally. However, when the display is viewed from a different angle, different pixels are viewed through the retarder elements and the image is corrupted. Off-axis viewers see a confusing image which is a random dot pattern. The PML may be made from liquid crystal and switched off to give a public mode.
Another method for making a switchable public/private display device is disclosed in JP3607272 (Toshiba 2005). This device uses an additional liquid crystal panel, which has patterned liquid crystal alignment. Different aligned segments of the panel modify the viewing characteristics of different areas of the display in different ways, with the result that the whole display panel is fully readable only from a central position.
In GB2413394 (Sharp), a switchable privacy device is constructed by adding one or more extra liquid crystal layers and polarisers to a display panel. The intrinsic viewing angle dependence of these extra elements can be changed by switching the liquid crystal electrically in the well-known way.
GB2418518 adds a guest host (dyed) LC layer with a patterned electrode to a standard thin film transistor (TFT) LC display. The dyed LC layer can be switched between an absorbing (private) and non-absorbing (public) state. The dye molecule absorption is dependent upon the incident angle and polarisation of light. For a given polarisation and orientation, the absorption of the dye increases with larger viewing angles resulting in low brightness at high angles (narrow mode).
WO06132384A1 (Sharp, 2005) discloses the use of an extra liquid crystal layer located between the existing polarisers of a liquid crystal display (LCD) panel. In this location, the extra switch cell can modify the greyscale curves for off-axis light. This provides a higher level of privacy for images than the techniques disclosed in GB2413394.
GB 2439961 discloses the use of a switchable privacy device constructed by adding an extra cholesteric layer and circular polarisers to a display panel. The cholesteric layer can be switched between a public (wide view) mode and a private (narrow view) mode that can provide 360° azimuthal privacy.
Adachi et al (SID 06, pp. 228) and Okumura (US20050190329) disclose the use of a hybrid aligned nematic (HAN) cell to provide a switchable privacy function. The HAN cells used by Adachi and Okumura are used in conjunction with an underlying image panel.
The above methods all suffer the disadvantage that they require the addition of extra apparatus to the display to provide the functionality of electrically switching the viewing angle range. This adds cost and particularly bulk to the display, which is very undesirable, particularly in mobile display applications such as mobile phones and laptop computers.
JP09230377 and U.S. Pat. No. 5,844,640 (Sharp, 1996) describe a method of changing the viewing angle properties of a single layer LCD panel. This is achieved for a Vertically Aligned Nematic (VAN) LC mode. Electric fields in the plane of the display panel are used to control how the LC material tilts in a pixel area. The number and orientation of different tilt domains within a pixel can be controlled by the in-plane fields. A pixel with several tilt domains will have a wide viewing angle; a pixel with one tilt domain will have a narrower viewing angle. The use of this method to vary the viewing angle of a display is described. However, the viewing angle of a single tilt domain of the VAN mode described is not sufficiently narrow to provide good privacy.
US20070040975A1 (LG Philips, 2005) discloses the design of an LCD panel in which the LC alignment is different in different sub-pixel regions of the display. This allows some sub-pixel regions to display an image to a wide viewing region, while the other sub-pixel regions allow light transmission only to the side viewing directions. These second types of sub-pixels are then used to reduce the overall contrast of the image being shown to the side viewer by the first type of sub-pixel, thereby obscuring the image information. A similar method to this, in which angled gaps in the pixel electrode, rather than differing LC alignment, are used to determine the azimuthal orientation of the LC director in different regions of the display, is published in SID '07 Digest pp 756-759.
A similar method is disclosed in US20070121047A1 (LG Philips, 2005) and associated paper SID '06 digest pp 729-731. In these schemes, the LC is uniformly aligned across the whole display panel, but some sub-pixel regions have an in-plane field applied to them, while other sub-pixel regions have an out-of plane field applied to them. This allows some sub-pixel regions to display an image to a wide viewing region, while the other sub-pixel regions allow light transmission only to the side viewing directions. These second types of sub-pixels can then be used to reduce the overall contrast of the image being shown to the side viewer by the first type of sub-pixel, thereby obscuring the image information
These methods have the disadvantage that not all the sub-pixel regions of the panel are used to display image data to the on-axis viewer, so the display loses both brightness and resolution compared to a standard LCD containing the same number of independent TFT switched sub-pixels, in which all are displaying information to the on-axis viewer.
US20060109224 (Au Optronics, 2005) describes an LCD panel in which the LC alignment is varied between two regions within a pixel, each region having an asymmetric viewing angle property, the two regions having opposite asymmetry. In the wide viewing angle mode, both sub-pixel regions are driven identically, the overall viewing angle property from both regions combined therefore being symmetric. In the narrow viewing angle mode, one region is driven with a different voltage to the other, resulting in an overall asymmetry for the display and an image which is obscured to the side viewer. However, this method also results in reduced display resolution and reduced brightness in the private mode.
In addition to schemes which rely on altering the manner in which the LC alignment or electrode layout of the panel are arranged in order to provide the view angle range switching, methods have been described whereby a software-only switch is applied to a standard LCD display to produce a private mode. These include US20040207594 (Sharp, 2003), GB2428152A1 (Sharp, 2005) and Rocket Software, Inc. (http://www.rocketsoftware.com). However, all of these methods necessarily reduce the quality of the displayed image to the legitimate viewer in the private mode.
JP 1999-11-30783 (Mitsubishi, 1999) describes an In-Plane-Switching (IPS) display, in which the brightness displayed to the on-axis viewer from any pixel in both modes (the grey level) is determined by the voltage between two interdigitated electrodes disposed on one of the cell substrates, which has an inherently very wide viewing angle. The display type is provided with a switchable privacy function by the addition of a third electrode on the opposing cell substrate allowing an out-of-plane field component to be applied to all pixels to generate an asymmetry in the brightness distribution. This method maintains nearly full brightness and resolution of the display in both modes, but it is only applicable to IPS type displays and it is believed that the privacy performance is not as strong as that from the type of display described here.
U.S. Pat. No. 6,646,707 (BOE Hydis, 2001) describes a fringe field switching (FFS) type display, in which both the pixel and counter electrodes are formed on the lower glass substrate of the display, overlapping one another and separated by a layer of transparent insulator material. The fringe field resulting from a voltage between the two electrodes extends into the bulk of the LC layer and is used to rotate the LC director, which is planar aligned parallel to the surface of the substrates, in the plane of the cell in order to control light transmission. No third electrode is used on the counter substrate and no variable viewing angle function is described.
A similar scheme to JP 1999-11-30783, which uses an FFS display, in which the two electrodes between which the grey level voltage is applied are disposed on top of one another on one cell substrate with an insulating layer between them, is described in US20060267905A1 (Casio, 2005). Again, a third electrode to allow switchable viewing angle properties is disposed on the opposite cell substrate. This scheme also, while maintaining the original display brightness and resolution, provides insufficient privacy strength. It also only describes a planar aligned LC device in which it is always the fringe field from the two electrodes on the lower substrate which controls light transmission.
A method in which switchable viewing angle is achieved for a Vertically Aligned Nematic (VAN) type display is disclosed in US20070046881 (Casio, 2005). This scheme uses two sets of electrodes on the substrate opposite the pixel electrode, one of which is registered vertically over the pixel electrode to produce a symmetric viewing angle range, the other set being offset to produce an oblique applied field and asymmetric viewing properties. This scheme however adds greatly to the complexity of the display panel and results in greatly reduced transmissive aperture in the display, thus reducing brightness.
US20070040780A1 discloses a number of schemes for providing a single layer LC panel with switchable viewing angle properties, including a method whereby a continuous pinwheel alignment (CPA) type display (also known as ASV) has the alignment arrangement of the LC director within each sub-pixel region altered to change the viewing angle properties of the pixel. However, the method outlined in the document, which uses only single layer electrodes on both substrates, has been shown to be problematic due to the gaps between the electrodes which provide the in-plane field disrupting the LC alignment in the public mode and degrading the wide viewing angle. This document also describes a method whereby a bias voltage is applied to the two halves of each CPA liquid crystal domain in the panel, creating an asymmetry in the brightness distribution with viewing angle. This effect has been shown to have a privacy performance which is not as strong as that of the device described here, however.
It is therefore desirable to provide an LCD display panel which is switchable between public and private viewing modes, requires no extra LC layer or other bulk-increasing components, and has brightness, resolution and quality of image equivalent to a standard public-mode-only LCD with the same number of TFT switching elements.